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Queue C

Date: 2023-11-15Last modified: 2023-12-22
CqueueTAILQ

Table of contents

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

// /usr/include/bsd/sys/queue.h
// /usr/include/x86_64-linux-gnu/sys/queue.h
#include <sys/queue.h>

From man queue:

The <sys/queue.h> header file provides a set of macros that define and operate on the following data structures:

// The data type for the node
struct node {
  char c;
  // This macro does the magic to point to other nodes
  TAILQ_ENTRY( node ) nodes;
};

int main( int arc, char *argv[] )
{
  // This macro creates the data type for the head of the queue
  // for nodes of type 'struct node'
  TAILQ_HEAD( head_s, node ) head;

  // Initialize the head before use
  TAILQ_INIT( &head );
  char string[] = "Hello World\n";

  printf( "Put some nodes in the queue:\n" );
  struct node *e = NULL;
  int          c = 0;
  for( c = 0; c < strlen( string ); ++c ) {
    printf( "Allocate memory for node %c\n", string[c] );
    e = malloc( sizeof( struct node ) );
    if( e == NULL ) {
      fprintf( stderr, "malloc failed" );
      exit( EXIT_FAILURE );
    }
    e->c = string[c];
    // Actually insert the node e into the queue at the end
    printf( "TAILQ_INSERT_TAIL %c\n", e->c );
    TAILQ_INSERT_TAIL( &head, e, nodes );
    e = NULL;
  }

  // print the queue
  TAILQ_FOREACH( e, &head, nodes )
  {
    printf( "TAILQ_FOREACH %c\n", e->c );
  }

  // free the elements from the queue
  while( !TAILQ_EMPTY( &head ) ) {
    e = TAILQ_FIRST( &head );
    printf( "TAILQ_REMOVE %c\n", e->c );
    TAILQ_REMOVE( &head, e, nodes );
    printf( "Release memory from node %c\n", e->c );
    free( e );
    e = NULL;
  }

  return EXIT_SUCCESS;
}

Possible output

Put some nodes in the queue:
Allocate memory for node H
TAILQ_INSERT_TAIL H
Allocate memory for node e
TAILQ_INSERT_TAIL e
Allocate memory for node l
TAILQ_INSERT_TAIL l
Allocate memory for node l
TAILQ_INSERT_TAIL l
Allocate memory for node o
TAILQ_INSERT_TAIL o
Allocate memory for node  
TAILQ_INSERT_TAIL  
Allocate memory for node W
TAILQ_INSERT_TAIL W
Allocate memory for node o
TAILQ_INSERT_TAIL o
Allocate memory for node r
TAILQ_INSERT_TAIL r
Allocate memory for node l
TAILQ_INSERT_TAIL l
Allocate memory for node d
TAILQ_INSERT_TAIL d
Allocate memory for node 

TAILQ_INSERT_TAIL 

TAILQ_FOREACH H
TAILQ_FOREACH e
TAILQ_FOREACH l
TAILQ_FOREACH l
TAILQ_FOREACH o
TAILQ_FOREACH  
TAILQ_FOREACH W
TAILQ_FOREACH o
TAILQ_FOREACH r
TAILQ_FOREACH l
TAILQ_FOREACH d
TAILQ_FOREACH 

TAILQ_REMOVE H
Release memory from node H
TAILQ_REMOVE e
Release memory from node e
TAILQ_REMOVE l
Release memory from node l
TAILQ_REMOVE l
Release memory from node l
TAILQ_REMOVE o
Release memory from node o
TAILQ_REMOVE  
Release memory from node  
TAILQ_REMOVE W
Release memory from node W
TAILQ_REMOVE o
Release memory from node o
TAILQ_REMOVE r
Release memory from node r
TAILQ_REMOVE l
Release memory from node l
TAILQ_REMOVE d
Release memory from node d
TAILQ_REMOVE 

Release memory from node

References